The ability of lasers to provide high intensity monochromatic light at a number of wavelengths has revolutionized the field of optics. In a number of modern optics applications it is important to be able to tune the output wavelength of the laser over a broad wavelength range or band. Tunable lasers have been developed to address this requirement. In particular, tunable external cavity lasers have found wide acceptance in various fields of optics as sources of tunable laser light. External cavity lasers derive their light from a lasing medium positioned within an external cavity typically delimited by two reflectors. Commonly, the mechanism for tuning the external cavity lasers includes a diffraction grating and a rotatable reflector that selects a desired wavelength from the light diffracted by the grating. The type of external cavity used by this laser is frequently referred to as a Littman cavity and its implementation for wavelength tuning of external cavity lasers have been described in numerous prior art publications, including Michael G. Littman et al., “Spectrally Narrow Pulse Dye Laser Without Beam Expander”, Applied Optics, Vol. 17, No. 14, Jul. 15, 1978, pp. 2224-2227.
The lasing medium provides gain over a wide wavelength range or tunable range, while the optical path length of the external cavity defines discrete longitudinal modes of light that are supported by the cavity in accordance with the standing wave condition. The light resonating within the external cavity can occupy any of those longitudinal modes provided that the gain of the lasing medium at the corresponding wavelength is sufficiently high. If the band delimited by the wavelength selection mechanism covers several adjacent modes, then the light obtained from the external cavity can contain wavelengths defined by these adjacent modes. Clearly, this is not desirable. Consequently, the mechanism for tuning the laser wavelength should be narrow band, i.e., it should define a narrow band containing just one of these longitudinal modes at any wavelength within the tuning range. The tuning should also be effectuated such that at all times the light derived from the cavity remains in one longitudinal mode. Such operation is frequently referred to as continuous single mode scan.
Unfortunately, in tunable lasers using Littman cavities simple rotation of the mirror does not provide for a continuous single mode scan over the tuning range. Specifically, wavelength discontinuities at which light skips or jumps from one longitudinal mode to another occur during the tuning process. Such jumps, commonly referred to as mode-hops, are difficult to control because during the tuning process the length of the external cavity and in particular its optical path length varies as well. In the publication of Michel G. Littman et al., “Novel Geometry for Single-Mode Scanning of Tunable Lasers”, Optics Letters, Vol. 6, No. 3, March 1981, pp. 117-118 the authors describe a cavity in which the mirror is translated axially as well as rotated to change the cavity length as well as the angle of the diffracted beam returned to the laser to avoid mode hops. Although the authors state that the pivot point selected by their method provides exact tracking for all accessible wavelengths, this is, in fact, true only for the case where there are no dispersive elements in the cavity, since the changes in optical path length due to the effects of dispersion are not considered. Further information of a general nature is available in Amnon Yariv “Introduction to Optics”, 1976, published by Holt, Rinehart and Wilson and Eugene Hecht, “Optics”, 1987, published by Addison Wesley Publishing Co. Further information on improvements to Littman cavities for tuned external cavity lasers can be found in Harold J. Metcalf et al., “Synchronous Cavity Mode and Feedback Wavelength Scanning in Dye Laser Oscillators”, Applied Optics, Vol. 245, No. 12, Sep. 1, 1985, pp. 2757-2761; K. C. Harvey et al., “External-Cavity Diode Laser Using a Grazing Incidence Diffraction Grating”, Optics Letters, Vol. 16, No. 12, Jun. 15, 1991, pp. 910-912, Guangzhi Z. Zhang et al., “Scanning geometry for Broadly Tunable Single-Mode Pulsed Dye Lasers”, Optics Letters, Vol. 17, No. 14, Jul. 15, 1992, pp. 997-999; U.S. Pat. No. 5,058,124; U.S. Pat. No. 5,319,668 and U.S. Pat. No. 5,802,085.
The tuning problems associated with Littman cavities as well as their size and delicate nature have prompted the introduction of other external cavity lasers, including ones in which the wavelength is selected by an interference filter positioned inside the cavity. For example, a laser with this configuration is described in the article by P. Zorabedian, “Interference-Filter-Tuned, Alignment-Stabilized, Semiconductor External-Cavity Laser”, Optics Letters, Vol. 13, 1988, pp. 826-828. A further adaptation of this approach to enable rapid tuning of the wavelength, such as required for wavelength-division multiplex (WDM) communication systems, is described in U.S. Pat. No. 6,108,355 to Zorabedian and U.S. Pat. No. 6,282,215 to Zorabedian et al.
Despite providing a more robust, smaller and more rapidly tunable devices external cavity lasers using interference filters still incur mode-hops during tuning. In addition, they are not sufficiently flexible and do not allow for rapid tuning with very small displacement of few optical elements.